As a number of computers are connected to the communication network and composing a system, each computer has become possible to be linked with the general public through the communication channel. Therefore, it is feared that even electronic information stored in a hard disk or other computer external memory storages be accessed by unauthorized outsiders through the communication channel, then stolen or altered.
Besides, electronic information comes often to be transmitted using a communication channel, as shown in exchange of personal informations such as electronic mail, credit card number, password, distributions of application programs such as game program, business program, and of data extracted for edit from a database.
In case of using a communication environment open to the outside for such electronic information exchange, an outsider who is not the receiver may acquire and use the electronic information during communication by interception or theft act. Especially, in case of distributing charged information or transmitting information involving the privacy, it is necessary to prevent electronic information during communication from being stolen easily.
Methods for securing the confidentiality of electronic information by the encryption are executed, because it is enough that the unrelated outside cannot use the electronic information even if it is acquired during communication or storage. The encryption technology developed for such effect exists variously for the encryption methods using symmetric keys, and for the encryption methods using asymmetric keys.
However, even when these encryption technologies are used, any person who would have acquired a decryption method by deciphering the encryption or by any means can restore easily and obtain useful information, as long as the whole information is contained in the stored electronic information or the transmitted electronic information. Moreover, the information may be altered or falsified, so we should always worry if the electronic information retrieved or received maintains the genuine information or not. Especially, the conventional method is uncertain, in case of storing or transmitting an electronic information requiring a high level of concealment, such as authentication data, high level personal secret information or company secret information and so on.
There, the inventor of the invention has already disclosed by PCT/JP99/01350 a method for dividing an electronic information file into a number of information elements, combining them in different orders to generate several information blocks and sending them individually or storing them in external memory storages individually. During the generation of information blocks, division/extraction data is created which holds information concerning the size of respective information elements, the order of combination of information elements and so on, the information blocks and the division/extraction data are sent individually or stored in external memory storages individually. When the electronic information file is restored, information elements in the information blocks are cut out, rearranged and bonded in the correct order.
According to this method, information held by an individual information block is nothing but a part of the whole electronic information, and but a gathering of divided fragments, whose information value is diminished even if the information block is stolen.
Nevertheless, in case where the division/extraction data was stolen together with some information blocks and the information elements in the information block could be rearranged correctly, at least a part of information would be known correctly by the interceptor.
On the other hand, the division/extraction data describes the position of each information element in the original electronic information file. Consequently, in case where the division/extraction data and any information blocks were stolen, the interceptor could know correctly which the position is in the original electronic information file that corresponds to the information element in the stolen information blocks, therefore, the interceptor can acquire a fragment of the useful information or a clue to infer the whole picture of the original electronic information.